The present invention is generally related to CMOS amplifier circuits, and more particularly to Class AB input stages and Class AB output stages useful in low voltage CMOS audio power amplifiers.
Audio power amplifier circuits are widely used in mixed-signal integrated circuit (IC) products. Typically, low voltage CMOS audio power amplifiers are used as they consume relatively low power and operate at a low supply voltage of portable electronic equipment.
Referring to FIG. 1, there is shown at 10 a schematic of a conventional CMOS audio power amplifier. This conventional design includes an input stage 20 having a simple differential pair of MOS transistors, and a differential to single-ended conversion circuitry, as shown in FIG. 2. The shortcoming of the structure is that the maximum output current is ITAIL, as shown at 30 in FIG. 3(a), FIG. 3(b) and FIG. 3(c), thus the slew rate condition will occur when a large fast transient is applied at the input of the MOS differential pair. The direct consequences are that the amplifier output is slew rate limited, and that the highest frequency that the amplifier can handle is inversely proportional to the output amplitude.
With regards to output stages, the output stages are typically classified into two categories depending on how the quiescent current is controlled, i.e. (i) without feedback loop (or, feedforward quiescent current control), and (ii) with feedback loop. Output stages having a feedback loop have reduced speed, which can degrade the phase margin of the signal path, and even create stability problems, due to the feedback loop.
There is a desire for a low voltage amplifier with an improved low total harmonic distortion (THD), hense, an improved signal/distortion ratio performance for a given quiescent current consumption, and which may operate at a reduced supply voltage. Specifically, there is a desire for an improved input stage having a much larger output dynamic current and hence a higher slew rate. There is also a desire for an Class AB output stage having a faster speed and a low crossover distortion at low supply voltage, whereby the circuit is stable to eliminate problems introduced by a quiescent feedback current control loop.
The present invention achieves technical advantages as a low voltage low THD CMOS audio (power) amplifier having a Class AB input stage and a low voltage Class AB output stage.
In the Class AB input stage, voltage buffers are provided to help drive each of the MOS transistors such that they can sink a large amount of current, such that the output current is virtually not limited. As compared to conventional differential pairs of MOS transistors, for the same quiescent current consumption, the present invention achieves a much larger output dynamic current, and thus a higher slew rate. In addition, the input stage of the present invention has a flat signal/distortion versus frequency characteristic, thus achieving a signal/distortion performance significantly better than the conventional design for the same current consumption.
The Class AB output stage of the present invention works at a reduced supply voltage while having a high driving capability, and has a low crossover distortion. This structure is always stable, without worrying about problems introduced by the quiescent feedback control loop. The output stage has a higher over-drive gate-source voltage for the output transistors, and thus has a higher driving capacity allowing the circuit to operate more suitably at low voltages which is desired in low voltage mixed-signal CMOS design, such as 1.8 volt or even 1.5 volt architectures.